docs: def prop2b(gm, pvinit, dt):
      """
      Given a central mass and the state of massless body at time t_0,
      this routine determines the state as predicted by a two-body
      force model at time t_0 + dt.
  
      http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/prop2b_c.html
  
      :param gm: Gravity of the central mass.
      :type gm: float
      :param pvinit: Initial state from which to propagate a state.
      :type pvinit: 6-Element Array of floats
      :param dt: Time offset from initial state to propagate to.
      :type dt: float
      :return: The propagated state.
      :rtype: 6-Element Array of floats
      """
      gm = ctypes.c_double(gm)
      pvinit = stypes.toDoubleVector(pvinit)
      dt = ctypes.c_double(dt)
      pvprop = stypes.emptyDoubleVector(6)
      libspice.prop2b_c(gm, pvinit, dt, pvprop)
      return stypes.cVectorToPython(pvprop)